1. Field of the Invention
The present invention generally relates to a howling detecting apparatus for detecting a howl caused by acoustic feedback between a microphone and a loudspeaker in an acoustic system and a howling canceling apparatus based on this howling detecting apparatus.
2. Description of Related Art
In a PA (Public Address) system of a concert hall and an acoustic system such as SRS (Sound Room System) that includes a loudspeaker and a microphone, a sound outputted from the loudspeaker is fed back to the microphone. Consequently, the closed-loop gain of a particular frequency sometimes exceeds the unit value one, thereby causing a howl. To cancel the howl of this type, several techniques are known.
In the first technique, a situation prone to howl is intentionally created during a rehearsal while monitoring frequency characteristics of particular points in the acoustic system. From the monitoring result, it is determined that a howl occurs if a peak frequency continues over a certain reference time above a certain reference level. According to this determination, a filter for suppressing the level of the frequency band including the peak is configured by means of DSP (Digital Signal Processor). This technique is disclosed in xe2x80x9cAutomatic Howling Detecting and Canceling System Based on DSPxe2x80x9d Tsuge et al., AES Tokyo Convention Preliminary Document 1995, pp. 112-155.
In the second technique, an impulse response of an acoustic system is measured, and an inverse signal component of howl caused by a voice fed back to the microphone is computed. To be specific, this inverse signal component is computed by convolution of the measured impulse response and the voice signal. The obtained inverse signal component is subtracted directly from an output signal to eliminate the howl. This technique is disclosed in Japanese Non-examined Patent Publication No. 56-30397.
However, the above-mentioned first technique requires to set the filter beforehand during the equipment installation. Besides, every time an environmental change takes place such as microphone relocation during the installation operation, the filter setting must be adjusted.
As for the second technique, the compensative component (namely, the inverse signal component) of a howl obtained from the measured impulse response is subtracted directly from the output signal, so that the impulse response must be measured with a fairly high accuracy. Otherwise, compensation error occurs, which leads to unintended distortion of the output signal. For the accurate measurement of the impulse response, an impulse waveform is generated in the monitoring mode beforehand to measure a feedback signal in the acoustic system. Still, a problem remains that the impulse response fluctuates with environmental changes. Especially, a large-scale hall for example involves a relatively long sound travel path, the transfer function frequently being fluctuated by temperature variation or partial air-flow variation. Hence, it is virtually impossible for large-scale halls to cancel howling with an inverse signal.
It is therefore an object of the present invention to provide a howling detecting apparatus capable of avoiding howling detection error and compensation error and to provide a howling canceling apparatus based on this howling detecting apparatus.
It is another object of the present invention to provide a howling detecting apparatus capable of instantly coping with environmental variations without requiring preset operations and to provide a howling canceling apparatus based on this howling detecting apparatus.
The inventive howling detecting apparatus is provided in a sound system containing a microphone, a loudspeaker and an amplifier for detecting howling which may occur by feedback of sound from the loudspeaker to the microphone. In the howling detecting apparatus, a measuring section measures an impulse response of the sound system to determine a time length of a decay portion of the impulse response. A detecting section detects an occurrence of the howling when the determined time length is longer than a predetermined reference time length.
Preferably, the detecting section analyzes a frequency spectrum of the decay portion of the impulse response to determine a frequency point at which the howling occurs.
Preferably, the measuring section measures the impulse response in situ based on an input to and an output from the amplifier which is disposed between the microphone and the loudspeaker so as to determine the time length of the decay portion of the impulse response on real time. In such a case, the measuring section measures the impulse response by time-sequentially computing a spectrum of the impulse response in terms of a ratio of a power spectrum of the input to a cross spectrum of the input and the output. Otherwise, the measuring section measures the impulse response by digitally processing the input and the output without computing spectra of the input and the output.
Preferably, the measuring section periodically measures an impulse response of the sound system at a predetermined time interval which is longer than the predetermined reference time length.
Preferably, the measuring section determines the time length of the decay portion of the impulse response in terms of a duration during which a decibel of the measured impulse response falls below a threshold decibel.
The inventive howling canceling apparatus is provided in a sound system containing a microphone, a loudspeaker and an amplifier for canceling howling which may occur by feedback of sound from the loudspeaker to the microphone. In the howling canceling apparatus, a measuring section measures an impulse response of the sound system to determine a time length of a decay portion of the impulse response. A detecting section detects an occurrence of the howling when the determined time length is longer than a predetermined reference time length, and further analyzes a frequency spectrum of the decay portion of the impulse response to determine a frequency point at which the howling occurs. An attenuating section attenuates a frequency component of the sound around the determined frequency point so as to cancel the howling.
Preferably, the measuring section measures the impulse response in situ based on an input to and an output from the amplifier which is disposed between the microphone and the loudspeaker so as to determine the time length of the decay portion of the impulse response on real time.
Preferably, the measuring section periodically measures an impulse response of the sound system at a predetermined time interval which is longer than the predetermined reference time length.
Preferably, the measuring section determines the time length of the decay portion of the impulse response in terms of a duration during which a decibel of the measured impulse response falls below a threshold decibel.
Preferably, the attenuating section comprises an equalizer connected to the amplifier for variably attenuating a frequency component of the sound in response to the determined frequency point.
In an normal state where no howl is generated, an impulse response of the sound system or acoustic system includes only responses of a hall and circuits. When a howl appears, the decay time of the impulse response gets longer, the waveform thereof changing conspicuously. At the rear portion or tail portion of the impulse response at this moment, the frequency component causing the howl is dominant. Thus, the occurrence of a howl is determined by monitoring the damping tendency of the impulse response waveform. In this case, even if the predicted accuracy of the impulse waveform itself is not so high, the occurrence of a howl can be detected with high accuracy.
According to the howling detecting apparatus of the present invention, the impulse response of an acoustic system is measured. If the time from starting this measurement to a predetermined damping level is longer than a predetermined time, namely, if the tail of the impulse response becomes relatively long, it is recognized that a howl has occurred. In this case, even if the impulse response is predicted comparatively rough, the occurrence of a howl can be recognized with high accuracy. In addition, according to this howling detecting apparatus, after detecting a howl by the above-mentioned method, a howling point is detected from the frequency component included in the waveform of the impulse response after the predetermined time. This allows correct prediction of the frequency point at which a howl is caused.
According to the howling canceling apparatus of the present invention, the frequency component of the above-mentioned howling point is suppressed in the acoustic or sound system based on the result of the howling point detection by the above-mentioned method, thereby canceling howling. As compared with the conventional method of adding an inverse signal to an output signal for compensation, this howling canceling apparatus involves less chance of causing error compensation, which less adversely affects other frequency bands, thereby implementing effective howling cancellation.
In the present invention, the occurrence of a howl is detected by monitoring the damping tendency of an impulse response waveform. From the frequency characteristic at the tail of the impulse response waveform, the howling point is obtained upon occurrence of a howl. By suppressing the frequency component around the obtained howling point, the howl is canceled. Consequently, as compared with the conventional method in which the inverse signal obtained from the impulse response is subtracted directly from the output signal, the predicted accuracy of the impulse response waveform itself need not be set so high. This allows application of a simplified technique of predicting the impulse response from the input/output signal of an acoustic system not flat in spectrum. The application of this technique eliminates the necessity for providing a special instrumentation mode, and allows real-time continuous prediction of the impulse response. Thus, the novel constitution eliminates most of the conventionally required presetting operations, and is capable of instantly coping with environmental variations.